This invention relates to deposition of diamond film on substrate arrays.
Techniques have long been known for depositing substances, such as layers of semiconductor material, using a plasma that is formed into a jet. For example, U.S. Pat. Nos. 4,471,003 and 4,487,162 disclose arc jet plasma deposition equipments which utilize a plasma for deposition of semiconductors and other materials. Ions and electrons are obtained by injecting an appropriate compound, such as a silicon compound, into an arc region, and a jet (or beam) is formed by utilizing magnetic fields to accelerate and focus the plasma. In recent years, equipment of this type has been used to deposit synthetic diamond. Superior physical and chemical properties make diamond desirable for many mechanical, thermal, optical and electronic applications, and the ability to deposit synthetic diamond by plasma jet deposition holds great promise, particularly if plasma jet techniques can be improved for this and other purposes. A plasma containing hydrocarbon radicals and atomic hydrogen can be obtained using electrical arcing, and the resultant plasma focused and accelerated toward a substrate so that polycrystalline diamond film is deposited on the substrate. Reference can be made, for example, to U.S. Pat. No. 5,204,144, assigned to the same assignee as the present Application, for description of an example of a type of plasma jet deposition that can be utilized to deposit synthetic diamond on a substrate.
In various commercial applications it is desirable to have relatively large size diamond films. In plasma jet deposition techniques there are various factors which limit the practical size of the deposition area that is active on a substrate at a particular moment. For example, when an arc is employed to generate the heated gas mixture in an arc jet plasma deposition system, the diameter of the beam can be limited by a number of factors. Since the cross-section of the plasma beam is generally limited in practical applications, the area on which it is desired to deposit a diamond film may be larger than the deposition beam. This means that it may be desirable to move the beam and the target substrate with respect to each other during the deposition process. This has been achieved by spinning the substrate during deposition, which helps to promote temperature uniformity over the substrate, as well as to attain larger area substrate coverage (see e.g. the referenced U.S. Pat. No. 5,204,144). Reference can also be made to U.S. Pat. Nos. 5,342,660, 5,435,849, 5,487,787, and 5,551,983, and to U.S. patent application Ser. No. 08/480,580.
In the typical DC plasma assisted CVD diamond deposition process, a DC arc jet is used to produce activated species of gases, often atomic hydrogen and hydrocarbons. These activated species are caused to flow over a substrate which is maintained at appropriate conditions of temperature and pressure to foster the deposition of polycrystalline diamond film. The percentage of the activated gas species that is engaged into the actual deposition process at the substrate surface is called the capture efficiency. This percentage is often quite low, such as under 50%, particularly when a single flat substrate surface is used.
In such cases, much of the activated gas species does not flow directly over the deposition surface, but is lost as overspray, diverted over the sides of the substrate, or does not penetrate the gas boundary layer which covers the substrate surface and hence is not available to participate in the actual film deposition process.
A second problem with single flat substrates is the non-uniformity of thickness of the deposited film, due to non-uniform gas conditions over the entire area of the substrate. Uniformity can be improved by methods such as those described in the above-referenced U.S. Patents and Application. There are limits, however, to the size of single diamond films which can be properly deposited because the properties of the gas change as the relative amounts of atomic hydrogen and activated carbon species in the plasma beam are depleted by deposition and by losses. For similar reasons, the relative quality of the deposited film is also subject to variation across the diameter (or linear dimension) of the substrate. Particularly, the amount and relative percentage of atomic hydrogen in the gas mixture tends to decrease as the gas continues to flow across the deposition surface. This can result in a undesirable change in the quality and growth rate of the diamond material.
It is among the objects of the present invention to provide improvements in plasma beam deposition of diamond film. It is also among the objects of the present invention to provide improved substrate arrays and techniques using such arrays in diamond film deposition.
The subject invention utilizes a plurality of deposition surfaces in a manner which: (1) improves the capture efficiency, which produces more diamond per amount of activated gas species; (2) improves the uniformity of activated gas species over the area of the individual substrates and among all of the substrate elements in the array, in order to improve the uniformity of thickness of the diamond film over the entire surface of each substrate element in the array, and among all elements in the array; and (3) improves the uniformity of activated gas species and the uniformity of the ratio of atomic hydrogen to activated hydrocarbon over the entire surface of each element, and uniformity among all substrate elements in the array, which tends to increase both the absolute quality and uniformity of quality of the deposited films.
One of the objectives hereof is to reduce the amount of atomic hydrogen wasted. Arc jet plasma jet diamond film deposition systems can be operated at low enough pressures ( less than 50 torr) that recombination in the gas phase is not an important loss mechanism for atomic hydrogen. However, hydrogen atoms are wasted when they recombine in some position other than on a part of the growing diamond that eventually ends up in a useful application, and the loss can be quite significant.
Enhancing the efficiency of gas capture will generally lead to larger substrate surface area for diamond film deposition. Increases in monolithic film size, however, normally leads to increases in yield losses from broken diamond as well as lifting prior to scheduled run completion. Broken diamond is even more likely as the thickness of the film decreases. Post deposition processing of production volumes will require standardized coupons (individual diamond film segments) for cost effective, automated manufacturing. In addition, advanced substrate systems will likely require three dimensional mandrel surfaces from which flat films are produced. These constraints all illustrate the desirability of employing segmented substrate surfaces in which the deposition area is divided into numerous standard sub-regions, sized to maximize the yield of unbroken diamond coupons.
The fact that carbon species both diffuse more slowly and are consumed at the substrate surface less efficiently than atomic hydrogen results in a gas stream which is decreasing in its ratio of atomic hydrogen to carbon as it flows along the surface of a substrate. For this reason the quality of the diamond deposited will decrease along the gas flow path in any substrate system which attempts to utilize only a long gas path to increase the usage of active species. In addition, the overall fraction of active species will decrease along the gas path, leading to reduced growth rates at the extremes of the substrate system. Therefore, in order to deposit uniform diamond films with efficient utilization of gas species one must take steps to ensure that all deposition surfaces receive similar exposure to the gas stream of the plasma beam. Two approaches hereof which help in achieving the stated objective are as follows: (1) moving of substrate surfaces such that all locations receive a similar time averaged exposure to the gas stream; and (2) design of substrate array geometry such that all surface locations experience a gas flow with substantially the same nominal deposition potential.
In accordance with one form of the invention, there is disclosed a method of depositing diamond film on a plurality of substrates, which comprises the following steps: providing a plasma beam containing atomic hydrogen and a carbonaceous component; providing a plurality of substrates, each of the substrates having a deposition surface, the substrates being arranged such that the beam impinges successively on a deposition surface of a first of the substrates and then on a deposition surface of a second of said substrates; the deposition surfaces of the first and second substrates being oriented with respect to each other at a non-zero angle. In a preferred embodiment of this form of the invention, the deposition surfaces are planar surfaces, and the non-zero angle is at least 10 degrees. The beam impingement on the deposition surface of the second substrate results in a reduction in boundary layer between beam and deposition surface.
In accordance with a further form of the invention, there is disclosed a method of depositing diamond film on a plurality of substrates, comprising the following steps: providing a plasma beam containing atomic hydrogen and a carbonaceous component; providing several substrates having respective deposition surface petals arranged in a symmetrical concave array; rotating the array; and directing the beam at a portion of the array, so that during rotation of the array, the surface petals are successively positioned for direct impingement by the beam. In a preferred embodiment of this form of the invention, the step of directing the beam comprises directing the beam substantially off axis with respect to the axis of symmetry of the symmetrical concave array. [As used herein, substantially off axis means that the beam center is offset from the axis of symmetry of the array by at least 25 percent of the array outer diameter.] In this embodiment, the beam directly impinges on no more than half of the surface petals at any time. The rotation of the array exposes all petals of the array to substantially the same time averaged flux of the atomic hydrogen and carbonaceous component of the beam.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.